Laser designators are vital components in today's military high precision targeting engagements. A laser designator operator surgically selects a target by placing a high energy pulse-timing coded laser beam onto a target. The laser beam on the target serves as a guide to a high precision, semi-active laser guided munition.
Laser designation requires typical pulse energies of tens of milijoules (mJ), pulse widths of tens of nanoseconds (ns), and precise pulse timing of the pulses in the pulse train. This last requirement means that the pulse-to pulse timing jitter should be very low. In order to generate bursts of tens of ns pulses with required pulse energy and precise timing, laser Q-switching is used.
Q-switching of solid state lasers is a known method for achieving high pulse energy in solid state or other types of lasers. Q-switching is achieved by placing a temporally variable or optical power dependant loss element inside a laser cavity. When the loss element is in a low transmission state, the laser action is blocked so that high population inversion can be built up in the pumped gain medium, corresponding to a large stored energy. When the loss element switches to a high transmission state, laser action commences, quickly releasing the stored energy via stimulated emission. The Q-switched laser output is relatively short and high energy (and high peak power), with a typical pulse-width of tens of ns, and energy of tens of mJ in a Nd:YAG laser (with a typical 5 mm×5 mm crystal cross-section).
Generally, there are two methods of Q-switching lasers. The first method, known as active Q-switching, relies on an electrically controlled loss element, such as an electro-optic crystal that rotates a polarization state of a transmitted light. The polarization rotation of the electro-optic crystal can be translated into a change of optical transmission with a polarizer on the same optical axis as the electro-optic crystal.
The second Q-switching method is known as passive Q-switching. This method uses a saturable absorber, such as Cr+4:YAG. The saturable absorber maintains a low optical transmission state when the laser gain medium is pumped until the round trip cavity gain becomes sufficient to overcome the cavity loss. This initiates laser action and a build-up of high optical fluence in the laser cavity, causing rapid bleaching of the saturable absorber, and generation of a high energy Q-switched pulse.
Comparing the active and passive Q-switching methods, each has distinct advantages and disadvantages. The active Q-switch laser is relatively large and costly but produces precisely timed pulses with low pulse-to-pulse jitter. The passive Q-switch laser is simple to implement, but has a large pulse-to-pulse timing jitter and lower overall laser efficiency. The timing jitter arises from a variety of factors, including thermal, spatial mode variations and pump noise. Previous efforts to reduce jitter include using a composite pump approach as described in J. B. Khurgin et al., Applied Optics 41, 1095-1097 (2002). Others have disclosed optical triggering but the implementation into a system is not very practical. See T. Dascalu et al., “Investigation of a passive Q-switched, externally controlled, quasicontinuous or continuous pumped Nd:YAG laser,” Opt. Eng. 35, 1247-1251 (1996) and X. Yin et al., “Actively-controllable passively Q-switched laser,” Proc. SPIE 5627, 199-208 (2005). Also, some have proposed direct bleaching of the saturable absorber. See U.S. Pat. No. 5,408,480 to H. Hemmati, U.S. Pat. No. 6,335,942 to Huang et al. and U.S. Pat. No. 7,324,568 to Spariosu et al. For both techniques, jitter reduction can be attributed to a rapid change induced in the laser cavity loss or gain, causing the Q-switching to occur at a precisely defined time. For the composite pump pulse, the Nd:YAG inversion was allowed to build to just below threshold, at which point a more intense pump pulse was added to rapidly increase the cavity gain. For the direct bleaching method, an optical pulse was used to bleach the saturable absorber, rapidly decreasing the cavity loss and driving the laser above threshold.